The invention pertains to the field of electronic device fabrication. More particularly, the invention pertains to a method of depositing films.
A common step in electronic device manufacturing is the deposition of a thin film over a substrate or over another film on the substrate. Multiple depositions contribute to the complexity and functionality of the electronic device. However, deposition of one material onto another frequently causes residual stress in one or both materials during fabrication.
Stresses Involved in Deposition
Thin films encounter residual stress, which determines the reliability of these films. Residual stress leads to failure of the films due to delamination, cracking or voiding. Both thermal stress and intrinsic stress contribute to residual stress. Thermal stress occurs when there is a difference in expansion between the materials as the temperature changes, thus causing strain between them. Intrinsic stress is caused by film growth processes. These stresses lead to destructive effects such as cracking and delamination of the film. This reaction to stress occurs in large part due to tensile stresses which are established in the outermost surface of the films. These stresses, should they exceed the fracture strength of the film, cause a fracture in the film. Decreasing these intrinsic stresses during deposition of materials is essential to reliable and economical film processing.
One type of stress addressed in the prior art is encountered during growth of materials, and is caused by lattice mismatch. This phenomenon occurs when the lattice structure of one material does not match the lattice structure of another with which it has direct physical contact. The differences in lattice structure cause mismatch, resulting in strain and growth-incurred defects.
Solutions have been presented to alleviate the problems encountered with lattice mismatch. For example, grading techniques are used to repair lattice mismatches in which intermediate buffer layers are placed between two layers which are not lattice matched to provide structures having reduced defects due to the mismatch. In addition, other layers with a graded composition can be used between two layers which are not lattice matched.
As an example, in U.S. Pat. No. 5,751,753 (Uchida ""753) and U.S. Pat. No. 5,901,165 (Uchida ""165), a buffer layer consists of a graded composition. The buffer layer lies on an underlying layer. The underlying layer has a lattice constant substantially different from an upper level semiconductor layer. The buffer layer has a lattice constant which gradually changes from the lattice constant of the first layer to that of the lattice constant of the second layer and relaxes lattice mismatch in a semiconductor laser. The gradual changes in the lattice constant of the buffer layer allow the upper level semiconductor layer to be grown on the buffer layer.
The Uchida ""753 and Uchida ""165 patents do not attempt to solve the problem of the unique stresses encountered during deposition onto a patterned substrate. The graded buffer layer discussed in Uchida ""753 and Uchida ""165 is designed to separate two layers of material which would otherwise encounter stress. In addition, Uchida ""753 and Uchida ""165 are not concerned with deposition onto a substrate that has already been patterned.
In U.S. Pat. No. 5,035,923 (Sarin ""923), a continuous deposition process is disclosed in which the reactant gases used gradually change from the reactants required for one layer of the coating to the reactants required for another layer. The coating disclosed in Sarin ""923 includes at least two layers. The first layer consists of aluminum nitride or aluminum oxynitride deposited over a silicon-based ceramic substrate as an intermediate layer. At least one outer oxide layer with protective and surface properties is deposited over the intermediate layer.
Although the Sarin ""923 patent discloses a stress resistant coating, the grading discussed is directed to the composition of the layer being deposited. The coating itself is a graded composition, including at least two different materials. This coating process is not designed to deposit a homogeneous layer of one material on a substrate. In addition, the issue of patterned substrates is not addressed in the patent.
A number of deposition parameters affect film stress, including system background pressure, substrate bias, gas ratio (noble/reactive), input power, the morphology of the substrate, the source to substrate distance, substrate temperature, type of magnetron source (fixed or rotating), gas delivery, pumping speed and loading mechanisms. In the case of pulsed DC deposition, a specific type of deposition whereby pulsed current is used during the deposition, additional parameters such as pulse width, magnitude, and frequency also influence film stress.
Though the list of parameters which affect stress can be extensive, at the core is how these parameters influence the kinetic energy with which the material""s ions and molecules strike the substrate surface. For example, lower background pressure increases the interaction distance between molecules and ions (Debye length), thus allowing an increase in their kinetic energy. This increase in the molecule""s impact energy causes the film to be compacted along the growth direction, resulting in a force directed perpendicular to the growth direction and tangential to the surface. If the tangential force is sufficiently high to overcome the adhesion of the layer to the substrate, the film detaches and curls to relieve the stress. Similar arguments apply to the effect of substrate bias, for example, where the applied electric field again serves to increase an ion""s kinetic energy.
Complicating the solution to the problem is the fact that film stress is a function of the material upon which it is grown. For example, stresses can vary by orders of magnitude depending upon whether aluminum nitride is grown on silicon or aluminum. For an unpatterned substrate with a homogeneous surface, known deposition parameters are used to control film stress. However, most applications require that the material be grown on a patterned substrate. For example, aluminum electrodes are frequently formed on a silicon substrate before depositing aluminum nitride. With such patterned substrates, low stress growth conditions for one material generally do not match those of the other material, thereby resulting in undesirably stressed films on one surface or the other.
Properties of Aluminum Nitride
One film material that has characteristics suitable for electronic device manufacture is aluminum nitride (AlN). Aluminum nitride possesses desirable electronic, optical, and thermal properties. Among the useful properties of aluminum nitride are its piezoelectric response, its high thermal conductivity, its close thermal expansion match to a silicon substrate, and its good mechanical strength. Therefore, successful deposition of aluminum nitride onto a patterned substrate is essential for further development of advanced electronic devices.
However, in order to deposit aluminum nitride on a patterned substrate, resultant film stress must be taken into consideration. None of the prior art addresses the problem of stresses encountered during deposition of a material such as aluminum nitride on a patterned substrate. A method to successfully alleviate stress on each component of a patterned substrate is needed in order to improve the deposition process.
The invention presents a deposition method which varies the growth conditions of a film on a patterned substrate. For example, deposition conditions required for obtaining growth are determined for each of the substrate""s component surfaces. Film deposition begins under the conditions used to deposit material on one of the substrate materials. Then, the growth parameters are adjusted towards desired conditions for the other substrate material as another small amount of material is deposited. The adjustment and subsequent deposition is repeated until the desired conditions for growth on the second material are met. Growth of the remainder of the film then continues under the desired deposition conditions for growth on the second material.